A SAS storage system shown in FIG. 1, for example, has been known as a storage system in which SAS standard communication is performed on the back-end (between a controller and a storage device) thereof (“SAS storage system” hereinafter).
The SAS storage system has duplex controllers (“CTL” hereinafter) 111, 111, and one or more enclosures 101. One or more enclosures 101 are coupled serially to the CTLs 111, 111.
Each enclosure 101 has a plurality of ports 121, a plurality of SAS expanders (“expanders” hereinafter) 123, and a plurality of storage devices 125. In the storage system, a communication path to each of the storage devices 125 is duplicated. One of the communication paths is coupled from one of the CTLs 111 to each of the storage devices 125 via one of the ports 121 and one or more of the expanders 123. The other communication path is coupled from the other CTL 111 to each of the storage devices 125 via another one of the ports 121 and any other one or more of the expanders 123. One or more of the expanders 123 and any other one or more of the expanders 123 are coupled to each other serially. A port 121 that is not coupled to either one of the CTLs 111 is sometimes coupled to a port of another enclosure (not shown). When data is written to the storage devices 125, the upstream side of the flow of the data (the CTL 111 side, for example) is referred to as “upper-level (UL)” and the other side as “lower-level (LL).”
When failure occurs in a certain expander 123 in one of the communication paths, the CTLs 111, 111 cannot access, through this communication path, the storage devices 125 that are coupled to all of the expanders 123 lower than this expander (“failed expander” hereinafter) 123. For this reason, the CTLs 111, 111 need to go through the other communication path in order to access the storage devices 125 that are coupled to the expanders 123 lower than the failed expander 123. However, this concentrates loads onto the expanders 123 in the other communication path.
In order to avoid this, a method for bypassing the failed expander is considered. Note that bypass technology associated with a storage system is disclosed in Patent Literature 1.
Patent Literature 1 discloses enclosures (ENC) coupled by a FC (Fibre Channel), each of which is provided with an ENC bypass for bypassing a FC loop, and a PBC (Port Bypass Circuit) for switching between whether to go through or to bypass the FC loop. More specifically, the ENC bypass is a bypass of a disk connection part. The PBC selects either the disk connection part or the ENC bypass through which data should be transferred. When the occurrence of failure is detected in the disk connection part, a controller is requested to bypass the disk connection part. Accordingly, the controller controls the PBC to switch a data communication path from the disk connection part to the ENC bypass.